The nocebo response
The pill may be inactive, but the side effects are real.
About 20% of patients taking a sugar pill in controlled clinical trials of a drug spontaneously report uncomfortable side effects — an even higher percentage if they are asked. These effects are one kind of nocebo — a word that means in Latin "I will harm," as placebo means "I will please."
A placebo makes patients feel better for reasons unrelated to the specific healing properties of the treatment. A nocebo makes patients feel worse (or does other harm) in the same way. Common symptoms are drowsiness, headache, mild dizziness, difficulty concentrating, and stomach upset. Many health professionals are not aware of nocebos, yet the reaction can cause patients to drop out of clinical trials, stop taking drugs they need, or end up using other drugs that complicate their treatment.